Well it depends on your sound card.
Have you got any information on your motherboard if its onboard sound? Or soundcard in the number of sound channels it has?
If its a 5.1 setup, you should have at the back, a stereo connection for left and right, a centre connection, a rear connection, and possibly a subwoofer connection. (Stereo is green)
For 6.1, you'd have another centre speaker at the rear to help for positioning of back sounds.
If you have a sound card that can produce 6.1 results, it might be best going for the full type
Though you probably wouldn't notice much difference in sound, though the extra rear channel might help for awareness of where the sound is coming from.
3D SURROUND PLUGINS/PROGRAMS:
Something I've got to say to. You can get many plugins for Winamp that allow for 3D sound so that music fills all these speakers in a realistic way, even if its just for echo purposes.
Usually only 2 speakers will be used without special sound types or formats for music.
PowerDVD if you buy the full setup also has somethnig called 'Dolby Pro-Logic-2'. This allows for all speakers to be used for msuic in a realistic fashion.
The voices would be at the centre, the instruments in stereo, and at the back you'd get extra sounds and echo that come out from a song, which is selective on how the sound sounds. This gives excellent impact
DIFFERENCE BETWEEN PRO-LOGIC 1-2:
Differerence between Prologic 1-2 by the way if you've heard of it is that 2's back speakers are in stereo
Though I think Dolby Pro logic 2 only supports 5.1 speakers, you should be able to find software to support 6.1
With DVD's though, you'd definitley pick Dolby Digital as the output function to give the desired effect.
As of price goes...finally
(they shouldn't be all that diff) the price for 5.1 is about $110-140 rounded from other retailers
6.1 is around $140-170 rounded from other retailers
Hope this helps
EDITED: added titles to make it easier